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A Case of the Shakes


Tracking down the source of onboard vibration requires a methodical approach.


Being out on the water presents a wonderful opportunity to de-stress, but too much vibration can shake your hardware loose and detract from your enjoyment.

We were reminded of this fact during a recent pre-purchase survey, when the surveyor noted excessive vibration and recommended a repair. After the purchase, the buyers took the boat to a repair yard, where the shaft was replaced. Unfortunately, the problem persisted, and they visited another repair facility that suspected the prop and removed it for reconditioning.

As you might have guessed, after the work at both yards, the problem remained. The owners’ story is a lesson in how challenging it can be to diagnose vibration problems correctly.


Before going down the rabbit hole of vibration mysteries, start by having a mechanic come on board for a test run. Boat owners can become desensitized to sound and vibration, and a technician with trained eyes and ears can uncover problems that an owner may not recognize.

In this case, on the trial run, there was no question something was up. Starting around 2500 rpm, a thrumming sound radiated from the drivetrain. This disturbance only increased the faster the boat went.

Before diving into more serious troubleshooting in this type of situation, a diver may need to be called or the boat may need to be hauled to verify the propeller condition. In addition to marine growth, the remains of a crab-pot line can also give a boat the shakes.


After hauling the boat, the hunt for the culprit begins with an

inspection of the prop. Before messing with the drivetrain, you need to eliminate the prop as a suspect. The most conclusive diagnosis involves removing the prop and sending it out for a digital scan.

If that process is unavailable or unaffordable, you can check for any gross variations without removing the prop. Using vise grips or a similar tool, clamp a metal rod to the rudder and adjust it so that the end barely grazes the edge of the propeller at its widest point. Then, rotate the prop to the next blade and compare the clearance. It should not vary by more than one- sixteenth of an inch. Repeat this process for each blade, and then move the rod out closer to the blade tips and repeat.

There are other, less obvious noises and vibrations props can make. There can be issues with two-legged struts and four- bladed props. Since two of the prop blades can be shielded on each rotation, a noticeable vibration can occur. You should have a space of at least 15 percent of the prop diameter between the prop’s tips and the hull. Less space than that, and the prop will make noise as it passes the hull.

If the vibration is new, these causes are not in play. If the vibration developed after removal of the propeller, then the prop might not be properly installed. If the prop is not installed properly, it can bind on the key, preventing it from fitting tightly on the shaft taper.

If you have any concerns about the skills of the installer, pull the prop and have it reinstalled.

In the case of our example, the prop had just been reconditioned and fitted properly on the taper, so further investigation was required.


A shaft can cause vibration in one of two ways. First, the shaft might be bent, either as a result of impact to the propeller, or because of mishandling during shipment or installation.

A bent shaft can often be detected while it’s still installed in the boat. Check it with a caliper while slowly rotating the shaft. The bend, however, might be at an inaccessible portion of the shaft, requiring removal for inspection.

The second way the shaft can cause vibration is through lack of adequate support. There are well-established protocols for the bearing support for each diameter of shaft. Lack of adequate support can lead to shaft “whipping.”

The builder or a subsequent repairer may have failed to install the correct amount of cutlass bearings, or may have allowed too much shaft to overhang the after cutlass bearing (only 1½ times the shaft diameter is allowed between the forward end of the prop and the closest support). The cutlass bearings may simply be worn out, allowing the shaft to bounce around.


Motor mounts are basically two metal plates with rubber between them. The plates are sometimes formed so that if the rubber fails, the two pieces of metal will remain attached. Engine manufacturers use different durometer rubbers (sometimes on the same engine) to isolate the normal engine vibration from the engine stringers. Manufacturers also give replacement intervals for the mounts, because worn or damaged mounts can transfer too much vibration to the boat.

If you’re lucky, the motor mounts are in good shape, not rusty or older than the manufacturer’s recommendation for replacement. Mounts often get neglected, sometimes to the point of the rubber becoming completely disconnected from the metal.

The motor mount under the raw-water pump is often the worst of the bunch. Raw-water pumps have a weep hole in the housing near the pump’s shaft. The weep hole shows any leaks: green indicates a seawater leak, while oil residue indicates a failing oil seal. A leaking raw-water pump will slowly but surely destroy the engine mount beneath it.

Motor mounts should be checked before a trial run to be sure that all the fasteners are tight. Another check should be done underway and under load while shifting the boat in and out of gear to be sure there is not too much movement.

Depending on how soft the mounts are, you shouldn’t see them flex more than about an eighth of an inch.


The shaft must align to the engine within very tight tolerances. A collar (referred to as the “coupling”) fits on the forward end of the shaft and bolts to a flange on the transmission. A coupling is a precisely machined steel component that transfers the rotational energy of the transmission to the shaft.

The American Boat and Yacht Council recommends that the face of the transmission coupling and the face of the prop shaft

coupling have no more than .004 of an inch in misalignment. Any more than that, and there can be excessive vibration, accelerated cutlass bearing wear and even transmission damage.

To perform a check of alignment, the shaft coupling will need to be unbolted from the transmission with the boat in the water. Then, a feeler gauge should be inserted between the two couplings’ faces to determine the gap. A feeler gauge is a collection of varying thicknesses of sheet metal that are bound and marked with each thickness. (Note that since this is a precision tool, any rust can ruin the readings. Feeler gauges need to be kept dry and well oiled.)

Check the gap by inserting individual gauge leaves between the two couplings until a leaf is found that has just a bit of resistance. Then, try inserting that gauge in at least four spots around the coupling (up, down, port and starboard). If the gauge feels exactly the same in all four spots, then you have perfect alignment.

This almost never happens. Either the gauge will be loose and the next-biggest gauge (or several steps bigger) will be necessary, or a thinner gauge will have to be found.

Whatever the difference between the thickest gauge and the thinnest gauge amounts to is your misalignment. For instance, if a .005-inch gauge fits in the top and bottom of the couplings, but no gauge fits on the port side and the starboard side has a

.011-inch gap, then the misalignment is .006 of an inch. Rotate the coupling and check again for consistency.

At this point, hope that no one has ever whacked the coupling face with a hammer. Slide the couplings far enough apart to inspect the faces. Four thousandths of an inch (our minimum for misalignment) is not very much, and a good hammer blow will easily ruin the machined face of the coupling.

Note that there are typically circular indentations and receiving depressions in the two coupling faces. There will be some shaft sag as the faces become decoupled. The weight of the unsupported shaft needs to be calculated and supported using a hanging scale. If the gap exceeds the goal, then the real work begins of moving the engine to bring it in line with the coupling.

Let’s assume that the mounts have been well maintained: the threaded portion has been kept free of rust and has been periodically lubricated. The locknut will be loosened, and the nuts on the mount can then be moved up or down as necessary to align the coupling in that plane.

If the coupling is out of alignment to port and starboard, then the engine mount’s foot bolts will be loose, and the engine will have shifted. Hopefully, the builder will have set the mounts in the middle of their adjustment, and we won’t run out of travel. If we do run out of adjustment, we may have to add incompressible shims or modify the engine stringers to give us more room. This process can take some finesse, muscle and gymnastics, depending on the engine room and size of the engine.

Motor mounts should be lightly lubricated, rust- free, and within their manufacturer’s service life span. Below: Prop shaft couplings transfer all the engine’s thrust to the propeller shaft. They must be precisely machined to fit each shaft

Parts should not be replaced unless they prove problematic, but finding and fixing the actual problem is worth it.

In some cases, an engine cannot be aligned because the shaft is misaligned with the shaft log or bearings. When that happens, the boat must be hauled, and if there are struts, they may have to be removed and reset. If there aren’t struts, then the shaft log may have to be removed and reset.

It is always good, in any case, to check the cutlass bearings and make sure they are not worn or loose. Many boats have cutlass bearings abaft the stuffing box that are not visible until the box and shaft are removed. Once at least .004 of an inch has been achieved and the coupling is ready to be bolted back together, torque your motor mount nuts, make sure the coupling bolts are Grade 8, use a thread-locking compound, and mark each torqued nut with a telltale marker.

To check for bent prop blades prior to a trip to the prop shop, clamp a rod to the rudder that just touches one blade. Spin the prop and watch the rod for any difference between blades.
The shaft coupling has been moved away for engine alignment, but this drivetrain includes a vibration dampening device. To properly check alignment, the vibration dampener must be removed.


There is a device between the engine and transmission that reduces the shock loads of shifting. This device is called a damper plate. It is in the bell housing between the engine and transmission. It utilizes either springs or rubber for mitigation, and once that material wears out or breaks, the damper plate can make an awful racket.

Most transmissions run relatively quietly unless there is a gear failure inside. It’s usually easy to pinpoint where the noise is coming from when this happens, and the source is typically going to be evident in gear at low rpm.


A variety of additional couplings can be installed between the transmission coupling and the prop shaft. These can be simple, flexible couplings (like a DriveSaver) that mount directly to the transmission between the two metal couplings. Or, they can be a more sophisticated constant velocity joint-type system that utilizes a short jack shaft and a bearing fixed to the hull. The constant velocity joint style allows for softer engine mounts and slight engine misalignment.

These systems can decrease vibrations at some added cost (sometimes, a lot of added cost) but they also can be another component that requires service, and that may fail. There is a lifespan to constant velocity joints, and to any rotating component that utilizes rubber or plastic. As these systems wear out, they can cause the vibrations and noises they are meant to suppress.

Flexible insert couplings, because their plastic faces are not machined, can’t be used to get a true alignment. The surface is not smooth enough to use a feeler gauge. Either a metal dummy coupling is used in place of the flexible coupling during alignment, or the prop needs to be pulled so the shaft can be pulled forward, and the shaft coupling brought to the transmission coupling. This method can’t be used as a final alignment since the boat typically has to be hauled. As solid as your boat feels in the water, the hull will change shape as it is brought onto land and blocked. Wait at least 24 hours after launching for the hull to retain its shape before completing a final alignment.


On the boat in our example, the propeller had been serviced, the shaft had been replaced, and both checked out fine. In addition, the alignment fell within the required ABYC standard. Further inspection ruled out any problems with the engine or transmission, and the culprit remained elusive.

Let’s return to the coupling on the forward end of the shaft. The coupling can be held to the shaft with a taper, a key and a nut (like a prop), or a straight shaft can be used with a key and set screws through the coupling to the shaft. There are also split couplings that use bolts to squeeze onto the shaft.

Having ruled out the other possibilities, we removed the shaft and sent it and the coupling to the machine shop. The machinist found that the coupling face wasn’t perpendicular to the shaft, a problem that induced a wobble and, hence, vibration. The lesson: Whenever a shaft is replaced, the coupling must be sent to the machine shop to make sure they match up.

Chasing down the cause of boat vibration requires a methodical approach and determination. Parts should not be replaced unless they prove problematic, but finding and fixing the actual problem is worth it.If you have been living with annoying vibration, you will be amazed and delighted when you head out on a boat that hums smoothly. Your boat will be more enjoyable, hardware won’t come loose, and you’ll start feeling those good vibrations.

Steve Zimmerman is the president of Zimmerman Marine, which operates five boatyards in Maryland, Virginia, North Carolina and South Carolina. Zimmerman has been building and repairing boats for more than four decades.

Max Parker is the operations manager of Zimmerman Marine. Parker has spent time in most of the trades in his over 25-year career working on boats and still loves to grab a tool bag and fix something when he gets the chance.